Morgan
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Flames Under Pressure
So after watching this, thinking about how they've done experiments with flames in microgravity, are there any videos of how flames behave in
increased atmospheric pressure? How or would a candle or match burn for example at 2.5 atmospheres? Would it be discernably different than at sea
level?
Opening a Soda on the Ocean Floor
https://www.youtube.com/watch?v=EJiUWBiM8HE
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happyfooddance
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I believe the flame would be more "contained", and probably "flutter" more rapidly. But I believe auto-ignition temp would be decreased, provided that
the pressurized atmosphere has plenty of oxygen to replace that which was consumed.
I have had a fascination with fire since I was a toddler. I have always studied flames, and to me, some of the most interesting and dramatic chemistry
happens in those interfaces where combustion is happening. The formation of radicals and highly energized species: is intense.
If you supercool a coke can (take it some degrees below 0°C), even after shaking it also will not be likely to erupt. Many of those fail videos with
erupting champagne bottles, I just assume the champagne was not cold, and the people opening them don't see the relationship between vapor pressure,
concentration, and temperature.
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Bert
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Increasing the pressure causes change in reaction rates sitar to increasing the % Oxygen. 2X pressure, 2X the oxidizer interacting with the burning
stuff- There is 2X more inert Nitrogen coming along with the Oxygen which takes some of the energy to heat up as well, but the net effect is a faster
burn. A fire in a caisson is a bad thing.
Somewhere, there are videos of an internal combustion engine running with glass cylinders.
 
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happyfooddance
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This is why I am a member of this forum.
Thank you.
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Bert
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Oh yeah! plenty of videos...
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happyfooddance
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Yes. I have been watching them for the last couple of hours. Thank goodness my son is asleep. When he wakes up he will have no choice than to watch
them too!
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RawWork
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Quote: Originally posted by Bert  | There is 2X more inert Nitrogen coming along with the Oxygen which takes some of the energy to heat up as well, but the net effect is a faster burn.
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There are books, blogs, tutorials claiming that the more wood stove is opened from bottom, the lower the temperature will be and more energy loss,
because more cold air will rush from house to fire, and more cold air will enter the house. But it's wrong because overall temperature will be higher,
fire uses/likes oxygen. Fire is different unlike almost everything else like heated object that doesn't burn. No matter how weak the thermal
isolation, no matter how cold the air, the more cold enters the house, the hotter. The only way to raise temperature.
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Morgan
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I'd like to see how pulsating combustion behaves with a simple jam jar jet too, running at ~2.5 times atmosphereric pressure or if there's anyone out
there that lives at a relatively high altitude, it would be interesting to know how a jam jar jet flame attempts to start or sustains at a lower
pressure. Maybe a larger or smaller than typical hole would help it out in slightly rarified air. Probably noone has run a jam jar jet in the dense
cool dry air in winter in Death Valley, as another extreme. Not that that would be anything but a slight variation in air pressure over STP but even
things like humidity and temperature can make an exponenial difference in how they run. I've jars that will run like a race horse in winter but won't
even sustain in summer in Florida for example.
As an aside, a tiny jam jar jet will run easily for ~10 seconds with not that many drops of alcohol (methanol or ethanol) for fuel and as such it
would be funny to see one on the space station if they would OK it for a demonstration on jets in space or something. Maybe a little screen in the
bottom of a vial jar would keep the fuel in place in microgravity or deisign the fuel to stay put by other means. Imagine a little vial jetting across
the cabin in micro gravity. Or tethered to a string or vectored thrust to go in circles.
Notice how little fuel is consumed in this tiny quartz vessel for ~15 seconds of run time.
Jam Jar Jet Mini Quartz
https://www.youtube.com/watch?v=VgMJac_amhk
Just a visualization below of some of the motion going on
"It's recorded in a way which makes it possible to see the relatively warm ejected gases"
Jam Jar Pulse Jet Schlieren
https://www.youtube.com/watch?v=q1LQ94pjUWE
I've read the space station keeps the pressure at the same as on earth. Oh well, just a left field thought that probably won't be happening anytime
soon. But I think jam jar jets would be fun presentation, a variation on flames in space if it worked.
"Normal air pressure on the ISS is 101.3 kPa (14.7 psi); the same as at sea level on Earth. An Earth-like atmosphere offers benefits for crew comfort,
..."
ISS ECLSS - Wikipedia
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sodium_stearate
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| Quote: Originally posted by RawWork | | Quote: Originally posted by Bert | There is 2X more inert Nitrogen coming along with the Oxygen which takes some of the energy to heat up as well, but the net effect is a faster burn.
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There are books, blogs, tutorials claiming that the more wood stove is opened from bottom, the lower the temperature will be and more energy loss,
because more cold air will rush from house to fire, and more cold air will enter the house. But it's wrong because overall temperature will be higher,
fire uses/likes oxygen. Fire is different unlike almost everything else like heated object that doesn't burn. No matter how weak the thermal
isolation, no matter how cold the air, the more cold enters the house, the hotter. The only way to raise temperature. |
It is true that when firing a steam locomotive, that one must
be very careful not to open the firebox damper too much because a point will very soon be reached where too much cold air gets pulled in and the
boiler pressure will suffer.
It's a critical balance of having just the right amount of
air draft to maximize the heat. Too much, or not enough
air both can cause it to run too cool.
"Opportunity is missed by most people
because it is dressed in overalls and it
looks like work" T.A. Edison
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RawWork
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| Quote: Originally posted by sodium_stearate |
It's a critical balance of having just the right amount of
air draft to maximize the heat. Too much, or not enough
air both can cause it to run too cool.
|
I think you're wrong. That's what my family thinks, same as you. I told them they're wrong too. Too much air cannot cool fire, but can only increase
it. It will only increase temperature. Fire "eats" and "likes" air. At least in closed stuff like furnaces where air gets supplied from below or side
of fire. Maybe in open space like outside some cooling can happen, but in furnace all air goes directly on fuel, and boosts fire. I am more often
complaining about fire too weak.
More precisely, it is critical balance, but wood is solid/heavier/denser, meaning larger reactant by mass. So it will never be too much air to cool
fire. Not even if it were liquid oxygen, as PeriodicVideos showed. Except when wood almost dissappears every half hour or so, depending on burning
speed, then we add more wood (reactant) to sustain reaction. But yeah, it's always wood that is in excess, and air that is lacking, so adding it just
moves proportions towards optimal/best/precise/maximum.
Equilibrium theory says the same.
[Edited on 7-3-2018 by RawWork]
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Bert
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| Quote: Originally posted by RawWork | | Quote: Originally posted by sodium_stearate |
It's a critical balance of having just the right amount of
air draft to maximize the heat. Too much, or not enough
air both can cause it to run too cool.
|
I think you're wrong. That's what my family thinks, same as you. I told them they're wrong too. Too much air cannot cool fire, but can only increase
it. It will only increase temperature. Fire "eats" and "likes" air. At least in closed stuff like furnaces where air gets supplied from below or side
of fire. Maybe in open space like outside some cooling can happen, but in furnace all air goes directly on fuel, and boosts fire. I am more often
complaining about fire too weak.
More precisely, it is critical balance, but wood is solid/heavier/denser, meaning larger reactant by mass. So it will never be too much air to cool
fire. Not even if it were liquid oxygen, as PeriodicVideos showed
[Edited on 7-3-2018 by RawWork] |
All right. You have a thought. Can you state this formally as an hypothesis, and then design experiment(s) to try and verify or disprove? Please do
recall, no matter what it feels like personally, an apparently negative result from an experiment aimed at proving your hypothesis is just as valuable
as a positive one.
I hope you will do these things and document the process here.
[Edited on 7-3-2018 by Bert]
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sodium_stearate
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In a typical steam locomotive firebox and boiler, the
draft is pulled up through the grates at the bottom of the
firebox. There is an ashpan under the grates in an enclosed
area. On the engine I personally was firing, this ashpan
has 2 openings, one at the front, and one at the back.
The front damper was usually left closed at all times except
when the first fire was being ignited early in the morning
to help it have enough air to properly kindle up into
a decent blaze.
During normal operation, the front damper remained closed
and the draft was controlled using the rear damper.
It was demonstrated to me again and again by the guy who
was training me about how to properly fire this old
wood-fired locomotive, that opening that rear damper
any too much when the locomotive was pulling hard
does more harm than good for the overall situation at hand.
Sure, you can open the damper wide open with the thought
that it will tend to burn the fire hotter. But what actually
ends up happening is that too much air burns the fire
too fast and it also blows holes right through a properly
thin fire. These holes then, allow cooler air up into the firebox
and then it gets sucked through the boiler flues where it
tends to cool the boiler rather than keep constant heat on it
like it should.
He demonstrated this to me time and time again.
He told me it's counterintuitive, and he's right, it is.
But he did have me open that damper a few times like I wanted
to, I even opened the front one a time or two. He told
me to watch the pressure gauge and see that it sags
down when that is done.
Where as, on the other hand, closing the damper down
so it's just barely cracked open, and then carefully placing
a few logs into the corners and other holes and thin spots
results in plenty of available pressure.
As he said "You cannot argue with a steam locomotive.
If you do, it will win every time"
Another thing which was said by another guy there
is that "These things run on hopes and dreams"
But that is a whole different topic. Back to the topic
here: In certain situations, too much air draft can and does
allow too much cool air up through the fire.
This is not speculation in the case of a steam locomotive.
This is documented and it can be freely repeated at any time,
by anyone who wants to see it in action.
"Opportunity is missed by most people
because it is dressed in overalls and it
looks like work" T.A. Edison
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Vomaturge
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I think everyone is partially right about drafts heating or cooling a fire. From my experience, it increases the speed things burn at (so more watts
of heat release rate) but also changes the heat transfer from one part of the fuel to another. I have seen parts of a half-burning charcoal light up
yellow and burn away in seconds when blown on, while leaving the remaining part cold. I think the effect of faster airflow is not exactly the same as
the effect of increased pressure and density without a change in flow velocity.
I have only played with high pressure fire once, a few years ago, and it was only glowing, not flaming. I wanted to see a flame in a higher partial
pressure of oxygen. I pierced a soda bottle cap and put a bicycle pump ball needle (like the one you use to fill football's) through the hole. I put
some moist wood chips in the empty bottle so I could drop burning stuff in without melting the plastic. Candles and burning sticks went out, or
threatened to melt through before I could screw the cap on. But I had a tiny piece of this smoldering rod, meant for lighting firework fuses. I think
it's called a "punk stick"? It looked like compressed or glued sawdust, and would smoke and glow for a very long time. At normal pressure, the glow is
not very visible in daylight, even on a cloudy day. I lit a piece of this rod and put it in the soda bottle on top of the damp wood chips. I couldn't
really see the glow, just the smoke. When I pumped air in through the cap, the airflow made it heat up and glow. When the sealed bottle reached ~4-5
atmospheres, the smouldering stick glowed even when the pump was not disturbing the air. There was no airflow in the bottle other than convection. It
smoked vigorously too, and seemed to dim as it used up some of the oxygen. I pumped the pressure up to 6 or 7 atmospheres, and it got even brighter.
The outside of the bottle got warm (yes, I know every part of this experiment was stupidly dangerous). The smoke got thick enough to block my view of
the burning rod. It was a very densely opaque white smoke (just what you would expect with 7 liters of it in a liter bottle). By the time I
depressurized it, the 2-3 cm of stick had been consumed. I didn't time it, but it seemed like just a few minutes. I didn't see it flame up at any
point. I had full length sleeves and glasses on, but no face shield. I don't even think I had earplugs or gloves on, but I can't remember for sure.
I'm probably pretty lucky the thing didn't explode on me. I haven't repeated the experiment since.
Edit: At the time, I thought the reaction was just getting hotter because it had more oxygen, but now I think the density of the combustion products
is just as important. The higher the pressure, the denser they'll be, and the greater their heat transfer back to the fuel. This makes more and more
of the fuel ignite, and also heats anything nearby more easily than if the hot gases were under less pressure. Here is a video demonstrating that even
materials that react exothermically without help from air still are affected by pressure:
https://m.youtube.com/watch?v=8Cx9mNnky2U
The pressure causes the combustion products to be denser, and hang near the fuel for longer, helping keep the thermal chain-reaction going.
Again, if you want to experiment with this phenomenon, do as I say, not as I do. Choose protective equipment, apparatus, and location so that it will
be completely okay if it bursts. Expect the pressure vessel to burst, and be ready to deal with that outcome. Remember that fire in a constant volume
sealed container will not only heat it severely, but also bring up the pressure further.
[Edited on 7-3-2018 by Vomaturge]
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Reboot
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Classic thermodynamics: Increase the concentration of reagents and you increase the reaction rate (because there are more opportunities for the
molecules of fuel and oxygen to bump into each other.) A classic demonstration of this is sticking a burning stick of wood or such into a jar that
has been filled with pure oxygen.
Whether increasing air flow helps the fire or not depends on how fuel-limitted the reaction is. If there was already plenty of air to burn the
available supply of fuel, then blowing more air into the reaction actually cools it as the additional mass of oxygen and nitrogen carries away heat.
(For instance, a gas burner is usually limited by the supply of combustable gas; blowing more air into the burner won't make it burn hotter, and may
blow it out!)
On the other hand, if the reaction conditions are able to provide more fuel than the current oxygen supply can burn, you can often burn it faster by
blowing more air in. For instance, a tightly contained coal fire in a forge is usually oxygen starved when not being supplied with 'extra' air, so
it flares up when you blow more air in with a bellows.
But...all things being equal, increasing oxygen pressure (whether as pure oxygen or from air) strongly TENDS to increase combustion rates. Until it
doesn't. :-)
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Sulaiman
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I suspect a little confusion here, so let me add to it 
Internal combustion engines compress the air and fuel VAPOUR,
efforts need to be taken to prevent detonation of the compressed (hence hot) gas mixture.
Increasing airflow to a fire will increase the HEAT output but not neccessarily the flame temperature.
A steam engine uses the exhaust steam to 'suck' air through the firebox by venturi action (or similar)
under these conditions, with limited fuel surface area, not all of the oxygen will be consumed,
at some airflow rate the cooling effect of the air could easily result in a significantly lower temperatures.
Why can you blow out a candle ?
CAUTION : Hobby Chemist, not Professional or even Amateur
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unionised
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| Quote: Originally posted by RawWork |
I think you're wrong. That's what my family thinks, same as you. I told them they're wrong too. Too much air cannot cool fire, but can only increase
it.
[Edited on 7-3-2018 by RawWork] |
If I was a member of your family, I would have lit a candle, then blown it out.
[EDIT I see I was beaten to it; never mind]
[Edited on 7-3-18 by unionised]
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RawWork
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Extremely interesting posts! I could argue lot more, but yeah experiment will quickly bring the truth. I will discuss that when i try that. Well, i
said that in some cases like in open space (house is like open space for candle) and if air is not blown from below or side of fuel i may be wrong.
But in candle you it's burning from above and is blown out from above. Plus it's special case where diffusion burns fuel slowly... For other posters,
you must be aware that fuel will always be hotter than air, otherwise air would glove and not fuel. There are liquids, gases, but i am talking about
solids like carbon/coal/wood. It is glowing. Forget about fire, glowing carbon at near end of wood is hotter the more you blow it. I am currently
testing all your claims, and video will be made soon.
[Edited on 7-3-2018 by RawWork]
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Vomaturge
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It is true that it depends on wether the fuel is a solid, liquid, or gas, and wether the burning is happening at the fuel's surface (glowing), or in a
cloud of material evaporated from it (flaming). It also depends on how much air there is versus fuel, and wether they are mixed prior to ignition or
mix as they burn. RawWork, I agree that fires are much more resilient to being blown out if the oxidation happens on the fuel surface, and the fuel is
the hottest reactant.
It's funny, I thought of the candle case before I saw it posted by sulaiman and unionised. In that case, the main thing burning is the vapor emitted
from the hot wick. Light a candle, then light another (or a lighter or something else) blow out the first candle, then within a second, bring the
still-burning flame into the residual smoke from the first candle that just went out.Hold it a cm or two above the extinguished wick. If it goes as
planned, a flame will propagate very rapidly down the trail of smoke, and relight the first candle. What's the relevance of this? The wick, and the
hot wax on the wick, hardly produce heat at all. They give off vapor, which reacts with the air, heating the wick enough to keep it emitting vapors.
Blowing on a candle probably mixes more oxygen into the fuel-poor core of the flame, and might actually cause a quick increase in temperature.
However, it moves it away from the wick, leaving no way for it to ignite the vapor there.
What I have noticed with glowing combustion (like charcoal) is that blowing on it sometimes makes the fire burn hotter in some areas, and uses up a
section of fuel quicker, but it doesn't always ignite the rest of the fuel. I think the issue is that fire (usually) needs not only a fuel and
oxidizer, but also heating where the two reactants meet. a breeze that blows out a candle or match might just drive the flame into a crumpled
newspaper or dry grass, igniting the whole thing. Anything that affects the heat transfer from the reaction into the unreacted fuel and air, will make
the fire burn faster and/or hotter. Blowing on a fire may either increase or decrease the heat transfer to the fuel; it depends on the situation. This
is one of the reasons gunpowder gently pyrolyses in a vacuum without producing much heat (as shown in the Cody'slab video), burns rapidly at
atmospheric pressure, and burns in milliseconds when confined in a gun barrel. Even putting it in an atmosphere of nitrogen will help force the hot
gases to stay near the powder grains, delivering extreme heat.
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Morgan
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Combustion Under Pressure - A New Understanding Revealed
"However, at even a moderately high pressure of 5 atm, wrinkles develop over the flame surface. The fact that the flame surface can become unstable
and develop wrinkles is not surprising. Indeed, as early as the 1940s, Russian physicist Lev Landau predicted that the flame surface is always
unstable. However, smooth flames such as those on a gas stove are the kind that is routinely observed, and the possible occurrence of wrinkled flames
has been treated as more of an exception than the rule."
http://www.spaceref.com/news/viewsr.html?pid=4051
I happened to recall this wrinkle-like effect in a whoosh bottle, although the pressure is not very great.
The best whoosh bottle ever || ViralHog
https://www.youtube.com/watch?v=gKkVWvpZYaY#t=5s
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